In recent years, a display device including a touch panel, which has a screen designed to be touched by a detection target such as a finger or a stylus and to detect the contact position, has been generally used particularly in the field of mobile devices such as smartphones and mobile phones.
Conventionally, as the touch panel to be included in such a display device, there have been mainly the following touch panels used: a resistive film touch panel (when the touch panel is pressed, an upper conductive board and a lower conductive board are brought into contact with each other, whereby the contact position is detected); or a capacitive touch panel (a change in capacitance at a touched position is detected, whereby the contact position is detected).
Of these, the capacitive touch panel has been mainly used recently, because it (i) is capable of detecting the contact position by simple operations and (ii) does not have to have two conductive films having an air gap between them like the resistive film touch panel and therefore the reflection of external light does not occur at a boundary between the air gap and the conductive film.
However, the capacitive touch panel has the following problem. That is, the capacitive touch panel is configured to detect the contact position of a detection target such as a finger by sensing a change in capacitance. If the touch panel receives radiation noise from outside, the noise eventually causes a change in capacitance. As a result, it becomes not possible to accurately detect the contact position.
FIG. 16 schematically illustrates a configuration of a display device including a general touch panel.
A display device 50 includes a liquid crystal panel 51, a touch panel 52 and a cover glass 53. The liquid crystal panel 51 and the touch panel 52 have an air gap between them. It should be noted that, in recent years, an air gap-free structure has also been increasingly used.
However, when the liquid crystal panel 51 is in operation, radiation noise occurs which adversely affects the operation of the touch panel 52 (see FIG. 16).
FIG. 17 illustrates how the amount of noise that the touch panel 52 receives changes depending on the presence of the liquid crystal panel 51.
As illustrated in FIG. 17, when there is the liquid crystal panel 51, the touch panel 52 receives much more noise as compared to when there is no liquid crystal panel 51. Therefore, the SN ratio (signal-to-noise ratio) decreases. This leads to a decrease in detection performance of the touch panel 52, and detection errors may occur more frequently.
Noise analysis was conducted on the liquid crystal panel 51. As a result, it was found that (i) the noise which is a cause of a decrease in detection performance of the touch panel 52 is radiation noise coming from the liquid crystal panel 51 and (ii) the radiation noise is being generated in the liquid crystal panel 51 while display data is being written, specifically, during a short period of time at the start of charging data lines.
In order to solve such a problem, Haga et al. have proposed the following method in Non-patent Literature 1 (SID 2010 DIGEST, pp. 669). In a display device including a surface-mount self-capacitance touch panel which is a kind of capacitive touch panel, the operations of the touch panel and a liquid crystal panel are synchronized with each other so that the touch panel is driven while no data is written to the liquid crystal panel. Contact positions are detected in this state.
FIG. 18 schematically illustrates a configuration of a display device 60 including the touch panel described in Non-patent Literature 1.
As illustrated in FIG. 18, the display device 60 includes a color filter substrate 61 and a TFT substrate 64. These substrates have a liquid crystal layer between them (not illustrated).
The color filter substrate 61 has, on its surface facing the TFT substrate 64, a color filter layer and an alignment film etc. (these are not illustrated) and has, on the opposite surface, a surface ITO layer 62 and a polarization film.
The surface ITO layer 62 has, at its four corners, four detection systems 63a, 63b, 63c and 63d which are constituted by voltage supply circuits VS1 to VS4 and current sensing circuits I1 to I4, respectively. The surface ITO layer 62 is configured to receive, at its four corners, voltages of the same level applied from the respective voltage supply circuits VS1 to VS4.
Under the circumstances, when a finger makes contact with the touch panel, a small amount of electric current passes through the finger via a capacitor Cr.
Depending on the position where the finger makes contact with the touch panel, different current values are sensed by the current sensing circuits I1 to I4 of the four detection systems 63a, 63b, 63c and 63d. On the basis of this, the contact position of the finger is detected.
Meanwhile, the TFT substrate 64 has, on its surface facing the color filter substrate 61, a plurality of pixel TFT elements 65, a gate driver (gate signal line drive circuit) 66, a data driver (data signal line drive circuit) 67, a common electrode Com and the like. The common electrode Com is electrically connected with a common electrode driver 68.
FIG. 19 illustrates timings of operation of the display device 60 shown in FIG. 18.
As illustrated in FIG. 19, the touch panel is configured such that it is driven to detect a touch during a V-blank period (during which the liquid crystal panel is not driven). During this period, both the output from the gate signal driving circuit and the output from the data signal line drive circuit are high impedance (Hi-Z).
That is, (i) the operation of the touch panel and the operation of the liquid crystal panel are synchronized with each other and (ii) the touch panel is driven and the contact position of a finger is detected during a period other than the “Addressable” period (see FIG. 19) during which data is being written to the liquid crystal panel.
As described above, by the driving method as described in Non-patent Literature 1, it is possible to prevent the effects of noise that occurs while data is being written to the liquid crystal panel.
On the other hand, Patent Literatures 1 and 2 disclose a configuration in which parallel driving is employed to reduce the time taken for the touch panel to carry out sensing.